PURPOSE: To illustrate the geometry for displacement current and to perhaps demonstrate displacement current.

DESCRIPTION: This is the classic displacement current experiment described in general physics textbooks. Displacement current is sensed as oscillating magnetic field between the plates of the capacitor. The oscillator is set to about 15 kHz, and tuned to give the maximum displacement current. Evidence of the displacement current is the existence of an azimuthal magnetic field between the capacitor plates. This is sensed by observing the EMF induced by inserting the search coil radially into the capacitor with the coil oriented vertically (photo at left). Holding the search coil in the capacitor parallel to the plates should produce considerably less pickup (photo at right).

This device can be used to demonstrate in three dimensions the geometry of the displacement current experiment. There is some discussion whether the actual pickup is due to displacement current or simply some sort of general electromagnetic pickup, that is obviously filling the area.

SUGGESTIONS:

REFERENCES: (PIRA 5C20.60) See Demonstration Reference File for a number of articles dealing with displacement current.

EQUIPMENT: Large parallel plate capacitor driven by oscillator, pickup coil (from demonstration M12=M21) attached to oscilloscope.

SETUP TIME: 10 min.